This invention relates to an improved closure or diaphragm assembly for offshore platforms and the like used in well drilling and production where it is desired to remove a portion of the closure or diaphragm from the member on which it is installed.
Offshore platforms are generally fabricated in a harbor or on a shore location and are then towed to a marine site where they are tipped on end and lowered into position with the platform resting on the ocean floor. The platform legs are hollow structures having open ends so that pilings can be driven downwardly through the legs into subterranean formations below the floor of the marine site to anchor the platform in position.
It is desirable during platform setting operations to utilize the platform legs and/or pile sleeves for buoyancy to assist in the setting operations. In some instances, buoyancy tanks may be included on the offshore platform to add buoyancy above that provided by the legs and/or pile sleeves to the platform. It is also desirable to exclude foreign material from the platform leg and/or pile sleeve during platform setting operations to prevent the annulus between the piling and the leg and/or sleeve from becoming contaminated with foreign material which would prevent the filling of the annulus with grout or cement. Therefore, a closure structure which is easily severable when the piling is driven through the platform leg and/or pile sleeve is used to seal the end of the leg or sleeve during the setting of the platform.
Typical prior art closures or diaphragms are illustrated in U.S. Pat. Nos. 3,533,241; 4,087,978; 4,124,988; 4,178,112; 4,220,422; 4,230,424; 4,367,983; and 4,470,726. However, as such closures or diaphragms are fabricated for use on offshore platforms in deeper and deeper water depths, of necessity, the number of fabric reinforcing plies must be increased to increase the load bearing capacity of the closure or diaphragm. As the thickness of the closure or diaphragm increases it becomes increasingly difficult to pierce the closure with the piling to be driven through the platform leg or pile sleeve by merely dropping the piling upon the closure. If it becomes necessary to use the pile driving hammer upon the piling to cause the piling to pierce the diaphragm, such use is dangerous because the initial hammer blow may be of sufficient strength to not only cause the piling to pierce the closure but, also, cause the piling to be driven many feet into the floor of the marine site thereby suddenly unloading the pile driving hammer. If the pile driving hammer is unloaded suddenly, before the hammer may be stopped, the repeated unopposed blows of the hammer may cause substantial damage to the derrick on the derrick barge installing the offshore platform.
In this connection, while it has been proposed to use various devices to initially pierce the closure or diaphragm before driving the piling therethrough, the use of such devices is not an attractive option because such devices must be transported to and from the marine site and easily assembled and disassembled during repeated use during platform installation.
As an alternative to such prior art closures or diaphragms, it has been proposed to use frangible plate or disc closures or diaphragms to close the legs and/or pile sleeves of offshore platforms. Such typical prior art frangible plate or disc closures or diaphragms are described in U.S. Pat. Nos. 3,474,630; 3,613,381; 4,212,563; and 4,322,181. However, such frangible plate or disc closures or diaphragms are not generally satisfactory because they are difficult to fabricate, install and use, particularly where the repeatability of rupture strengths are desired.
Yet other types of releasable closures are described in U.S. Pat. Nos. 4,024,723; 4,142,371; 4,175,592; 4,183,698; 4,373,835; and 4,376,597.
However, the releasable closure described in the U.S. Pat. No. 4,024,723 patent is generally undesirable for use because the annular cutter used to pierce the diaphragm may be retained upon the end of the piling during the driving operation thereby causing the piling to deflect rather than driving straight.
The releasable closure described in the U.S. Pat. No. 4,142,371 patent is generally undesirable because it requires the molding and maintenance during molding of an annular wrapped continuous cable in a large amount of elastomeric material to form the releasable member. Even if the cable can be controlled during the molding process, after the removal of the closure from the platform leg and/or pile sleeve, a large residue of elastomeric material is retained on the leg or pile sleeve upon which a piling may foul upon insertion into the leg or pile sleeve.
Also, the closure described in the U.S. Pat. No. 4,376,597 patent may be difficult to install for satisfactory use as the elastomeric closure or diaphragm must be subjected to uniform forces retaining it between its annular retaining rims, otherwise, it will tend to pull free or pop-out from the retaining rims upon loading.